As portable electronic devices, such as video cameras, cellular phones, notebook computers, etc., become more lightweight and have increasingly improved performance, research into batteries used as power supplies for such portable devices is being conducted. In particular, rechargeable lithium secondary batteries are being actively researched as they have three times as much energy density per unit weight compared to conventional lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, etc., and can be rapidly charged.
In a lithium ion battery, transition metal compounds such as LiNiO2, LiCoO2, LiMn2O4, LiFePO4, LiNixCox-1O2 (where x=1 or 2), Ni1-x-yCoxMnyO2 (where 0≦x≦0.5) and 0≦y≦0.5) oxides thereof containing lithium can be used as a cathode active material and a lithium metal, a lithium alloy, a carbonous material, a graphite material, etc. can be used as an anode active material.
Electrolytes can be classified as liquid electrolytes and solid electrolytes. When a liquid electrolyte is used, many safety problems, such as a risk of fire due to leakage of the electrolytic solution and breakage of the battery due to vaporization of the electrolytic solution arise. To solve these problems, a solid electrolyte has been proposed for use instead of a liquid electrolyte. Solid electrolytes do not leak and can be easily processed. Much research has been conducted into solid electrolytes such as polymer solid electrolytes. Currently known polymer solid electrolytes can be classified as complete solid electrolytes containing no organic electrolytic solution and gel-type electrolytes containing an organic electrolytic solution.
Since a lithium battery is generally driven at a high operating voltage, a conventional aqueous electrolytic solution cannot be used. This is because lithium contained in an anode and an aqueous solution vigorously react with each other. Thus, an organic electrolytic solution in which a lithium salt is dissolved in an organic solvent is generally used in a lithium battery. Such organic solvents should generally have high ionic conductivity, a high dielectric constant and low viscosity. However, since it is difficult to obtain a single organic solvent satisfying all these requirements, a mixed solvent may be used including, for example, an organic solvent with a high dielectric constant and an organic solvent with a low viscosity.
The carbon of an anode and an electrolyte in the lithium secondary battery react with each other during initial charging so that a passivation layer such as a solid electrolyte interface (SEI) film is formed on a negative electrode surface. The SEI film enables the battery to be stably charged and discharged without further decomposition of the electrolytic solution (J. Power Sources, 51 (1994), 79-104). The SEI film also acts as an ion tunnel through which only lithium ions pass, and prevents cointercalation of an organic solvent, which solvates the lithium ions and moves with the lithium ions into the carbon anode, thereby preventing a breakdown of the anode structure.
However, during initial charging, gas is generated inside the battery due to the decomposition of a carbonate-based organic solvent when forming the SEI film. This can lead to swelling and an increase in battery thickness (J. Power Sources, 72 (1998), 66-70). When the lithium battery is stored at high temperatures after being charged, the passivation layer gradually breaks down due to increases in electrochemical energy and thermal energy over time, the anode surface is exposed, and the amount of gas generated increases. The generation of the gas results in a local variation in adherence between electrode plates that results in the deformation of an internal battery and thus an excessive voltage is generated, thereby degrading the efficiency and stability of the battery. Also, since the solvent decomposes, the amount of electrolyte decreases, the electrolyte in the battery depletes and sufficient ions cannot be transferred, reducing the efficiency of the battery.